Seed destruction when using a stripper front – does it work

Take home messages

• Stripper fronts collect high levels of annual ryegrass seed, similar to draper fronts and therefore their use for cereal crop harvest will not negatively impact HWSC systems.
• The reduced levels of chaff produced during harvest with a stripper front influences the operation of HWSC systems.
• The weed seed collection ability of stripper fronts on other weed species is not yet known.

Background

Harvest weed seed control (HWSC) has been developed for use in Australian crop production systems as an alternative weed control technique that targets weed seed during grain harvest. Prompted by the widespread occurrence of herbicide resistance in major weeds of cropping systems, particularly annual ryegrass (Lolium rigidum Gaud.), these HWSC systems have been widely adopted by Australian growers (Walshet al. 2017; Kondinin-Group 2020). The majority of Australian grain growers are now using a HWSC technique to target weed seeds during harvest. When included in a weed management program, HWSC acts as a preventative weed control practice by targeting weed seeds to reduce weed seed inputs to the seedbank and therefore, future weed problems.

HWSC is effective on weed species where substantial proportions of seed production remain attached to plants at a harvestable height at the time of crop maturity, ensuring that grain harvest is also weed seed harvest. The potential susceptibility to HWSC of a weed species can be assessed by quantifying the degree of seed retention at crop maturity. An initial study assessing HWSC potential in Western Australian (WA) wheat crops identified high seed retention for the major weed species: annual ryegrass (85%), wild radish (Raphanus raphanistrum L.) (99%), brome grass (Bromus spp.) (77%) and wild oats (Avena spp.) (84%) (Walsh and Powles 2014). This geographically wide survey of weed seed retention in commercial wheat crops confirmed that high proportions of the total seed production of these species could potentially be targeted with HWSC systems. A number of studies have subsequently identified seed retention levels for numerous weed species at crop maturity, indicating their HWSC potential (Bitarafan and Andreasen 2020; Borgeret al. 2020; San Martínet al. 2021; Schwartz-Lazaroet al. 2021a, 2021b). Seed retention values reported in these studies were based on an assumed crop harvest height where crop (and weed) plant material were collected and processed during grain crop harvest.

The introduction of stripper fronts for, primarily cereal crop harvest, represents a major change in the collection and processing of crop grain (and weed seeds) during harvest. The action of stripper fronts is to collect (pluck) only the grain containing “heads” of cereal crops for subsequent processing and grain collection. This focus on the collection of grain heads greatly improves the efficiency of harvest by eliminating the need to process straw material that is collected when harvesting with a conventional draper front. With a substantial reduction in collected crop plant material, it is also likely that there is a reduction in weed seed collection and therefore, HWSC efficacy. Therefore, the aim of this study was to identify the level of weed seed collection during cereal crop harvest with a stripper front and the subsequent impact on HWSC.

Methods

Comparison of stripper and draper front weed seed collection

Annual ryegrass seed collection during harvest with stripper and draper fronts during wheat crop harvest was compared at two locations near Marrar, NSW (34.8333°S, 147.3667°E) in December 2017. To determine annual ryegrass seed production, prior to harvest at each location 20 representative annual ryegrass plants were collected. Each plant was harvested by cutting and collecting in a paper bag all plant material above 15cm. Remaining plant material was collected and placed into a separate paper bag. In each collected sample, the number of tillers and the average number of seed on each tiller was determined by threshing then cleaning the sample to separate out the seed which was then counted. For each plant, average total seed production and the percentage retention above 15cm harvest height was determined. Seed retention above 15cm was used as the estimate of weed seed collection when using a draper front operating at a common harvest height of 15cm.

To determine seed collection by stripper fronts, 50 annual ryegrass plants were located within each wheat crop. The number of seed-bearing tillers were counted, and plants were marked with spray-on road paint and the GPS locations recorded. After the area was harvested using Shelbourne® stripper front, marked plants were relocated and all remaining annual ryegrass plant material was cut at the soil surface and, along with any tillers on the soil surface, placed in a paper bag. Any plants which were driven over by the harvester wheel were noted. Samples were dried, weighed and the number of viable seeds determined as described above.

Chaff production

Six 100m lengths of wheat crop were marked out and, using the same Case® 8240 harvester, the first three lengths were harvested with a Shelbourne® stripper front, and the remaining three with a Case® draper front. A large shade cloth bag was attached to the chaff-line chute at the rear of the harvester. As each length of wheat crop was harvested, the chaff produced from each length was collected in the shade cloth bag and then transferred into a wool pack for storage. The average wheat yield obtained in each 100m length was recorded from the yield monitor to correlate with chaff production. The chaff produced from each length was weighed.

As there was a significant site effect (P<0.05), one-way ANOVAs were used to compare the effects of harvester front on annual ryegrass seed collection at each site. Lsds (p=0.05) were used for means comparisons. Due to the unbalanced nature of the design (draper n=20, stripper n=50), an ANOVA was run to try subsetting. However, the results remained unchanged, so subsetting was not used. Analysis of chaff production values was performed using a one-way ANOVA and means were compared using Lsds (p=0.05).

Survey of stripper front weed seed collection

Annual ryegrass seed collection during wheat crop harvest with a stripper front was recorded at eight locations near Wagga Wagga, NSW (35.1082°S, 147.3598°E). Prior to harvest, at each location, 20 representative annual ryegrass plants were located within the wheat crop. The number of seed-bearing tillers on each plant were counted, and plants were marked with numbered plastic tags. To collect any seed shed/dislodged during harvest, aluminium trays (20cm x 10cm) were placed around the base of the plant to cover the area of the plant canopy. To determine the average number of seeds per plant, an additional 10 plants were collected by cutting at ground level and placing in a paper bag. These plants were then oven dried at 70°C for 48 hours, seed bearing tillers were collected and counted, then individually threshed with the seed collected and counted.

After crop harvest, where a stripper front was used, marked plants were located and all remaining above-ground plant material was cut at the soil surface and collected, along with any dislodged tillers or branches on the soil, then placed in a paper bag. Material in aluminium trays was collected and sorted to retrieve any annual ryegrass seed. Plant samples were oven dried for two days at 70°C, weighed and the number of viable seeds determined, as described above.

Average seed production per tiller and seed-bearing tiller counts were used to estimate the seed production per plant on each of the 20 marked plants. The post-harvest seed counts per plant were then used to determine the amount and proportion of seed removal during harvest.

Results and discussion

Comparison of stripper and draper front weed seed collection

The use of stripper and draper fronts resulted in similarly high levels of annual ryegrass seed collected from mature plants present in wheat crops at harvest. Annual ryegrass seed collection was high at both Marrar sites for both stripper and draper front harvest (Table 1). At Site 1, there was no difference (P>0.05) in annual ryegrass seed collection between stripper and draper fronts – they both resulted in the collection of 91% of seeds. At Site 2, draper front collection was 94%; however, stripper front collection was lower (P<0.05), at 66%. The difference in seed collection at this site is believed to be due to a higher than 15cm harvest height used during harvest with the stripper front. The grower at this site was using a greater harvest height due to concerns about the presence of rocks during harvest. Higher harvest heights have previously been shown to result in lower weed seed collection (Walshet al. 2018). Growers should consider harvest height when using stripper fronts as well as draper fronts, as running the front lower will collect more weed seeds.

Table 1: Proportion of annual ryegrass seed production collected at two sites and the amount of chaff production during wheat harvest with stripper and draper fronts at site 1. Different letters indicate significant differences between values within columns (P<0.05).

Chaff production

When harvesting a wheat crop with the same harvester, the use of a stripper front produced almost half as much chaff compared to harvesting with a draper front. The observed large difference in chaff production may in part be due to poor harvester setup and operation that resulted in substantial amounts of crop stem and leaf material collected by the draper front exiting in the chaff fraction (Table 1). Regardless of the harvester setup and operation effects, there will likely be reduced amounts of chaff produced by the harvester when a stripper front is used, which will in turn influence HWSC systems.

With reduced amounts of chaff and no straw material collected during harvest with stripper fronts, there will be positive and negative impacts on HWSC, depending on which HWSC option is being used. If a stripper front is used, narrow windrow burning would not be possible due to the remaining standing stubble and the difficulty in burning chaff only windrows. Without straw residues, the use of bale direct systems will also not be possible. Chaff carts may be more efficient when combined with stripper front use, as there will be less chaff to collect, and fewer dumps to make. Although, the burning of these chaff dumps may be more difficult due to the lack of straw and resulting aeration. Use of stripper fronts may reduce chaff lining and chaff tramlining effectiveness. Weed seedling emergence is suppressed with increasing amounts of chaff material concentrated in chaff lines and the equivalent of more than 40t/hais required to completely prevent emergence (Walshet al. 2020). Therefore, as stripper fronts will produce less chaff material, the effectiveness of chaff and chaff lining systems in suppressing weed seedling emergence will be reduced. Impact mill operation would be more efficient with a stripper front compared to a draper front. There is a direct relationship between the quantity of chaff and the mill’s power requirements (Guzzomiet al. 2017).

Survey of stripper front weed seed collection

Wheat crop harvest with a stripper front provided similar levels of annual ryegrass seed collection as when a draper style harvester front was used. Across eight survey sites, high proportions of annual ryegrass weed seed collection were observed when wheat crops were harvested using a stripper front. The proportion of seed collected during harvest averaged 85% and ranged from 65% to 94% (Table 2). These values are equivalent to the seed retention values previously recorded for annual ryegrass in studies where there was an assumed draper front harvest at 15cm height (Walsh and Powles 2014; Borgeret al. 2020). Clearly then, the seed collection levels during a stripper front harvest are equivalent to those that occur during harvest when a draper front is used.

Table 2: Annual ryegrass average tiller, seed production on plants in wheat crops at maturity and the proportion of seed collected from these plants during 2020 wheat crop harvest with a stripper front at nine locations near Wagga Wagga, NSW. Numbers in brackets are standard errors for the mean of 20 replicates.

Conclusion

Assessment of annual ryegrass seed collection during wheat harvest with stripper fronts indicates that there is comparable seed collection to draper front crop harvest. Clearly, at least for annual ryegrass, the use of stripper fronts will not negatively impact the collection of seed during cereal crop harvest. The use of stripper fronts does reduce the amount of chaff material produced during harvest, which will impact on the types and potential efficacy of some HWSC systems. For example, there will be less material for impact mill systems to process that should lead to increased system efficiency. In contrast, lower levels of chaff material will diminish the suppressive effects of chaff lining on weed seedling emergence.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

References

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Borger CPD, Hashem A, Gill GS (2020) Comparison of growth, survivorship, seed production and shedding of eight weed species in a wheat crop in Western Australia. Weed Research 60(6), 415-424.

Guzzomi AL, Ryan M, Saunders C, Walsh MJ (2017) Reducing the integrated Harrington Seed Destructor power requirements through chaff fraction optimization. Applied Engineering in Agriculture 33(5), 695-701.

Kondinin-Group (2020) Harvest weed seed control — weed seed warriors. Kondinin Group No. 121

San Martín C, Thorne ME, Gourlie JA, Lyon DJ, Barroso J (2021) Seed retention of grass weeds at wheat harvest in the Pacific Northwest. Weed Science 69(2), 238-246.

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Walsh M, Rayner A, Ruttledge A, Broster J (2020) Influence of chaff and chaff lines on weed seed survival and seedling emergence in Australian cropping systems. Weed Technology 35(3), 515-521.

Walsh MJ, Broster JC, Aves C, Powles SB (2018) Influence of crop competition and harvest weed seed control on rigid ryegrass (Lolium rigidum) seed retention height in wheat crop canopies. Weed Science 66(5), 627-633.

Walsh M, Ouzman J, Newman P, Powles S, Llewellyn R (2017) High levels of adoption indicate that harvest weed seed control is now an established weed control practice in Australian cropping. Weed Technology 31(3), 341-347.

Walsh MJ, Powles SB (2014) High seed retention at maturity of annual weeds infesting crop fields highlights the potential for harvest weed seed control. Weed Technology 28(3), 486-493.

Contact details

Michael Walsh
Werombi Road, Brownlow Hill NSW 2570
0448 847 272
m.j.walsh@sydney.edu.au